Individual die on a device wafer may be individually capped by mounting an opposing cap wafer to the device wafer. Accordingly, each device on the device wafer mates to an opposing cap on the cap wafer. FIG. 1A schematically illustrates a device wafer 100 with a plurality of dice (individually, a “die” 105) arranged in rows 102R and columns 102C. Between the rows and columns of mesas are gaps, which may be known as “saw streets” 106.
FIG. 2A shows a corresponding cap wafer with caps 203 arranged in corresponding rows 202. Each cap 203 may be formed by etching a cavity 207 (see FIG. 2B) into the surface of the cap wafer 200, and by etching trenches (not shown) between the cavities 207, so that mesas 204 are formed. Each such mesa 204 has a top surface 205 that is what remains of the original surface of the cap wafer 200 surrounding the cavity 203. The dashed lines in FIG. 2A and FIG. 2B illustrate the outline of a corresponding die 105 on an opposing device wafer 100, and do not illustrate physical features of the cap wafer 200.
An intermediate layer, such as a glass frit 206, is formed between the die 105 and cap 203 to seal components of each die 105 within its cap 203 when the cap wafer 200 is mated to the device wafer 100, as illustrated in FIG. 3A.
In some applications, a part of a die 105 (such as a MEMS structure 104 shown in FIG. 1B and in FIG. 3A, for example) is within a volume 208 defined by a mesa 204 on the cap wafer 200 and the cap wafer 200 (e.g., the cavity 207), the device wafer 100, and intermediate layer 206, while other parts of the die (such as integrated circuitry and bond pads, for example) are outside of that volume in an area illustrated in FIG. 1B as hatched area 103.
Post processing, such as wafer thinning or removal of excess wafer cap material for example, may be performed on a capped wafer before the dice 105 are singulated. To protect the circuit elements 103 that are not enclosed within a cap 203, wax (not shown) is typically deposited between the device wafer 100 and the cap wafer 200, around and between the caps 203. However, the wax may be difficult to remove.
Prior to cutting the wafer into individual die (a process which may be known as “singulation,” or “singulating” the wafer), portions of the cap wafer 200 that are not covering a die (or a desired portion of a die) may be removed. Removal of such excess portions of the wafer cap 200 is often done by sawing the cap wafer 200 above the saw streets, as illustrated in FIG. 3B. In such a process, the saw blade 210 is set deep enough to cut through the cap wafer 200, but shallow enough to avoid contact with the device wafer 100. When the dice 105 are in a grid pattern, the excess wafer cap material may also be in a grid, and therefore be difficult to remove.
After the excess cap material and wax are removed, the dice are singulated by cutting through the device wafer along the saw streets